It was not the time of death of Abraham Lincoln, John F. Kennedy or Martin Luther King

It was not the time when the atom bombs were dropped, either on Hiroshima nor on Nagasaki

It is about aesthetics; the hands are symmetrically placed, which makes the clock look nice (and not broken or unbalanced – something you’d probably not want from a clock)

It is also about practicalities; the hands are not on top of each other, on top of the company logo, or on top of any date displays or other extra features (however the last one can be disproved by several of the above-depicted clocks – e.g. the Calvin Kline clock would show more extra features if it was set to 09:15.)

It is also to make the clock look happier, you could achieve the above by setting it to 08:20, but then the clock would seem like it’s frowning

That’s all good and well… but… the digital clocks…? None of the above reasons are valid for them… other than the fact that setting them to 10:10 or similar follows some company policy. Timex sets their clocks to 10:09:36 exactly, just to mention one such company policy.